Plural part core with exposed wound-core portion



April 64 H- c. HANSEN 3,129,399

PLURAL PART COREYWITH EXPOSED WOUND-CORE PORTION Filed Feb. 11, 1960 2 Sheets-Sheet 1 28 mmvroa 22 Hans Chi-{553:1 Hansen BY Ey mm M A TTORNE Y5 April 14, 1964 H. c. HANSEN PLURAL PART CORE WITH EXPOSED WOUNDCORE PORTION 2 Sheets-Sheet 2 Filed Feb. 11, 1960 INVENTOR. Hans hristan Hansen BY 4% 1w A TTORNEYS United States Patent 3,129,399 PLURAL PART CORE WITH EXPOSED WOUND-CURE PORTION Hans Christian Hansen, 14 Christianshoimsvej, Klarnpenhorg, Denmark Filed Feb. 11, 196i), Ser. No. 8,185 30 Claims. (til. 336-212) The present invention relates to a magnetic apparatus and more particularly to electrical coils having magnetic cores, such as choke coils, stray-field transformers or the like.

It is a main object of the invention to provide an electrical coil structure which has reduced electrical power losses improving its electrical efficiency.

It is a further object of the invention to provide a new and efficient electrical coil having an interior magnetic core and an exterior magnetic core of strip material facing the inner core with its broad side.

Still a further purpose of the invention is to provide a coil structure including a magnetic core in the form of an exterior hollow structure of strip material in a plurality of layers which faces the cavity of the exterior structure with their broad sides and in which a magnetic core carrying one or more coil windings is disposed within the cavity of said exterior magnetic core.

Still a further object of the invention is to provide a structure as specified hereinabove in which decreased losses can be obtained.

Still a further object of the invention is to provide a structure as hereinbefore specified in which good heat transfer to the surroundings is secured so that reasonably low operating temperatures can be obtained.

Still a further object of the invention is to provide a completely sealed coil structure.

Still a further object of the invention is to provide a coil structure useful as ballast coil or transformer in connection with luminous tubes.

Still a further object of the invention is to provide a coil structure of reasonably small dimensions.

Still a further object of the invention is to provide a coil structure having an exterior core portion in the form of a coiled strip of magnetic material which surrounds the coil as a container and also provides the exterior magnetic path and which is designed for mass production.

Further objects, features and advantages of my invention will appear from the following description of the invention with reference to the drawing illustrating only a few embodiments of the invention and wherein:

FIGURE 1 is an exploded view illustrating the parts of a coil structure according to the invention,

FIGURE 2 is a longitudinal section of the coil assembly along the lines 2-2 in FIGURE 3, and

FIGURE 3 is a section through the lines 33 in FIG- URE 2.

The coil structure according to my invention includes an induction coil in which windings of insulating wire are supported on an inner core section of magnetic material. The inner core 10, which preferably is a laminated core section, forms a central leg across the cavity or interior space of an outer core portion 12. The core portion 12 is made of strip material and comprises a plurality of laminated strip layers all having their broad sides facing the interior hollow space. The exterior core portion 12 surrounds the coil and protects it as a container and simultaneously it provides the exterior magnetic path defining a mantle. Slots 16, 18 and 16a, 18a are provided in the strip layers of the exterior core member 12, preferably in the orbit of the end surfaces of the central core section which are in physical contact with ICC the interior layer of the outer core or mantle structure 12.

According to one embodiment of my invention, which is not to be taken as limiting the invention, the outer core structure is made out of a continuous coiled strip of magnetic material and the slots 16 and 18 extend from opposite edges.

Seen from another aspect my invention comprises a coil structure including an exterior magnetic core structure 12 having a plurality of superimposed layers of strip material and defining an interior cavity preferably of substantially rectangular configuration between opposite ends of which an interior laminated core in of magnetic material extends supported at its end between the interior layers of the outer core structure. The induction coil winding 14 is supported on the interior core member 10. The strip width of the exterior core structure 12 corresponds substantially to the exterior dimensions of the winding 14 so that the interior core structure It) with the winding occupies the substantial part of the cavity defined by the outer core structure 12, the opposite open sides of which are closed by cover members 2i) and 22 of magnetic material.

Now, with more specific reference to the FIGURES 1-3 of the drawings, an induction coil structure is illustrated which comprises a coil winding 14 which may have one or more separate windings. The coil winding or windings are of insulated wire. Several layers of suitable insulating material, such as acetate sheet, may be interposed between the layers of the coil winding and corresponding layers may be interposed between the interior of the coil windings 14 and an inner core section 10 of any suitable magnetisable material, for example silicon steel. The inner core section is preferably a laminated straight core, either made by means of a plurality of layers stacked one upon the other, or by means of a strip folded in accordion fashion to form a laminated core with low hysteresis losses. The coil winding or windings 14 surround the inner core along the substantial part of its length and leave the end surfaces of the inner core 19 exposed.

The coil structure further comprises an outer core section 12 which provides the magnetic path and surrounds the inner core as well as the coil winding or windings 14 in the form of a mantle inside of which the inner core It) forms a central leg which extends between opposite inner surfaces of the outer core 12. The outer core 12 is in the form of a plurality of superimposed layers, i.e. in the form of a laminated structure of strip material which is disposed with the broad sides facing the interior space of the outer core.

Seen from another aspect the outer core is in the form of a tubular structure made of strip material defining a wall structure having an interior space which encloses the inner core 10 as well as the coil winding 14. This Wall structure hereby serves the double purpose of protecting the coil winding or windings 14 and forming part of the magnetic path together with the inner core 19.

As shown in the drawing, FIGURES 1-3, the outer core structure 12 or tubular wall structure is in the form of a substantially rectangular elongated member having side Walls 12a and 12b and end Walls 13a and 13b, the inner core 16 being disposed to extend between the end walls 13a and 13b with its end surfaces in physical contact with the inwardly facing broad side of the inner layer of the end walls, or if desired, separated therefrom by suitable air gaps.

In the orbit of the end surfaces of the inner core 1t) slots are provided in the layers of the outer core or mantle as exemplified by the slots 16 and 18 at one end and 16a, 18a at the other end.

In the embodiment illustrated these slots are in alignment with each other in each layer of the mantle so as to define channels or slots extending completely through the outer core structure from its exterior surface to the end surfaces of the inner core. These slots are contributing to decrease of losses as experienced according to my invention. By way of example in a core structure according to my invention designed and used as a ballast coil for a luminous tube and designed for a consumption of 40 watts, I have found that suitable provisions of the slots can decrease the losses from about 12 watts to in the neighbourhood of 7-8 watts, i.e., approximately 50%. I have found that it is not absolutely necessary to provide slots in all layers of the mantle structure, and neither is it necessary to arrange the slots in alignment, though throughgoing slots in alignment give the best result and are also the most convenient seen from a manufacturing viewpoint.

It has not been completely scientifically elucidated, what are the reasons for obtaining the advantage by means of the slots, but I believe that the slots partly provide an increased resistance in the magnetic path, where eddy currents are induced in the layers of the outer core, and in addition to this the slots serve as air gaps at the ends of the inner core member through which the magnetic flux is more evenly distributed to the layers of the outer core, whereby saturation of the inner layer or layers of the outer mantle is avoided. This also contributes-I believe-4o an improved distribution of the heat in the layers of the outer core and thereby to an improved heat transfer to the surroundings resulting in decreased operating temperature. I have thereby found that I can decrease the operating temperature of the coil. In many cases such decrease of temperature is not necessary, but seen from another aspect in the case of a maximum temperature allowed during the operation of the coil I can decrease the size of the coil and the amount of material used for the core structure, thereby aving weight, costs and space which is important in all kinds of electronic equipment where such coil structures can be used. As examples of use I have already mentioned ballast coils for luminous tubes. As other examples of fields where my novel and improved coil structures can be used I want to mention input or output transformers for radio and television sets and other sorts of transformers used in such equipment.

In the specific embodiment illustrated the outer core structure is made from a continuous strip of material wound on a form but it will be understood that my invention is not limited to that specific embodiment though winding of the exterior mantle 12 in the form of a loop enables a simple manufacturing technique as will be more fully explained in the following.

When winding the exterior mantle in the form of a continuous strip 1 have found it convenient to provide the slots as illustrated in a direction across the longitudinal direction of the ribbon. In the embodiment shown the slots in each layer of the exterior core structure 12 extend transversely of the ribbon. In the embodiment shown two slots, 16 and 18, at one end, as well as 16a and 18a at the other end are used, extending from opposite edges of the ribbon. The slots may extend between /3 and across the ribbon and are spaced from each other in the longitudinal direction of the ribbon. I have found that the distance between the slots is not very critical,-though I prefer that both slots, when two slots are used, are provided at each end of the interior core member 10. It is within the possible scope of my invention, however, to use more than two slots and also to let the slots extend in other directions than perpendicular to the edges of the strip or band.

Preferably the width of the strips or the ribbon from which the exterior core structure 12 is made corresponds substantially to the diameter or thickness of the coil 14 in a plane parallel with the sides 12a, 12b of the exterior core structure.

Hereby the core structure substantially fills the entire interior space within the exterior mantle or tubular wall structure.

According to another aspect of the invention the core structure is entirely enclosed by means of cover members closing the open ends of the tubular structure.

According to the embodiment illustrated in FIGURES 1-3 a pair of generally parallel cover plates 20 and 22 are arranged at opposite sides closing the space encircled by the exterior core or wall structure 12. Before closing the core structure entirely the cavities left between the coil and the exterior walls may be filled up with a suitable compound preferably a compound having good heat transfer properties so as to allow the heat to be distributed from the interior of the coil to the walls thereof. As example of such material polystyrol or suitable types polypropylene should be mentioned, but it will be understood by those skilled in the art that also other compounds are suitable within the scope of the invention.

As illustrated in FIGURE 1 the cover plate 20 has a configuration corresponding to the exterior configuration of the core mantle 12, while the other cover plate 22 has flaps 24 and 26 extending from each end for mounting an insulating terminal board 28 which may be secured to the flaps by screws, rivets or the like at the end most convenient for the mounting, or in the event that more than one coil winding is included in the winding 14 such terminal boards may be mounted at both ends.

As shown in FIGURE 1 supply leads 15 from the coil winding 14 extend through the channel defined by the slot 18.

The cover plates may be secured to the outer core structure by bonding, for example by means of suitable adhesive of. the type which hardens by means of a catalyst. As an example of such adhesive the one known under the trademark Araldite should be mentioned. The covers may also be secured by means of a strip of a suitable adhesive wound around the entire core structure. It will be appreciated that the cover plates 22 and 20, which preferably also are of magnetisable material, should be separated from the edges of the exterior core member 12 by an air gap so as to prevent losses in the form of eddy currents passing from the exterior core to the covers. The covers may also be secured to the outer core structure by means of spot welding. In such event the welding spots should be positioned so that the electrical connections between the exterior core member 12 and the cover plates through the welding spots do not cause any short-circuiting of the interruptions of the eddy currents provided by means of the slots 16, 18 and 16a, 18a in the event where these slots are perpendicular to the direction of the strip material and extend to the edges thereof. As apparent from FIGURE 1 of the drawing .the pairs of welding spots 30a, 30b and 32a, 32b between the top cover and the exterior core are located at the same side of the slots 18 and 18a while the corresponding welding'points 34a, 34b and 36a, 36b between the lower cover and :the exterior core 12 are located at the opposite, i.e. the right hand side of the slots 16 and 16a. The welding spots should be so provided that the electrical connection is only made between the cover plates and the exterior layer of the outer core structure. This can easily be done by placing the welding electrodes close together when the top cover is secured and it is convenient to provide apertures 38a and 38b in the flaps 26 and 24, each aperture having edge portions located where the cover plate would end, if the flap did not extend therefrom, so as to facilitate the correct location of the welding electrodes.

ous modifications may be possible within the scope of this invention.

Thus, for example, with respect to the core structure, the slots in the exterior mantle may vary in number,

and the slots may be arranged forming an acute angle with the edge of the ribbon,

While I have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of many changes and modifications within the scope and spirit of the present invention, and I therefore do not wish to be limited to the embodiment described only for purposes of illustration, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. An induction coil structure comprising in combination: a coil winding, an elongated straight laminated primary core member extending through the interior of said coil winding and having exposed end surfaces, a secondary core member surrounding said primary core member and said coil winding as a mantle, said secondary core member being of substantially rectangular configuration having side Walls and end walls defining together a tubular member having a hollow space in which said primary core extends between opposite end walls, said secondary core member being composed of a plurality of layers of strip material facing the interior of said tubular space with their broad sides, slots in each of said strip layers in the orbit of the junction between said ends of said primary core member and said secondary core member effectively providing means operable to limit eddy currents in said strip layers to flow substantially only within zones thereof between said slot, the slots in the different layers of said secondary core member being substantially in alignment, said layers of said secondary core member having a strip width corresponding, substantially to the exterior dimension of said coil Winding, and closure members of magnetizable material arranged adjacent each open end of said tubular member to provide a completely closed coil structure.

2. An induction coil structure comprising in combination: an electrical coil winding, an elongated straight laminated primary core member extending through the interior of said coil winding and having exposed end surfaces, a secondary core member in the form of a coiled band of magnetizable material defining a tubular laminated mantle having an exterior wall encircling a hollow space, the coiled band facing said space with its broad side, means supporting said electrical coil winding and said primary core with the latter extending between inside and across said mantle, slots in each layer of said mantle in the orbit of the junction between said primary core member and said secondary core member, said slots extending from the edge of said band transversely to the direction thereof, said band width corresponding substantially to the exterior dimension of said electrical coil winding, closure members of magnetizable material arranged adjacent each open end of said tubular mantle to provide a completely closed coil structure, and spot weldings between said closure members and the exterior coil of said mantle band disposed at one side of each slot only.

3. In an induction coil structure in combination: a first straight laminated magnetic structure, an electrical coil winding surrounding said first magnetic structure leaving its ends exposed, a second magnetic structure encircling said first magnetic structure and said coil, said second magnetic structure comprising a winding of a plurality of turns of a continuous fiat band-like magnetizable material wound upon itself to form a plurality of superimposed laminations facing said first magnetic structure with their broad sides, said first magnetic structure facing the inner lamination of said second magnetic structure with its exposed ends, at least a part of the lamination of said second magnetic structure having portions of reduced cross sectional area in the orbit of the junctions between said first and said second magnetic structures for causing an increase of the resistance against electrical currents induced in each lamination of said second magnetic structure.

4. In an induction coil structure in combination: a first straight laminated magnetic structure, an electrical coil winding surrounding said first magnetic structure leaving its ends exposed, a second magnetic structure encircling said first magnetic structure and said coil, said second magnetic structure comprising a winding of a plurality of turns of a continuous flat band-like magnetisable material wound'upon itself to form a plurality of superimposed laminations facing said first magnetic structure with their broad sides, said first magnetic structure facing the inner lamination of said second magnetic structure with its exposed ends, at least a part of the lamination of said second magnetic structure having portions of reduced cross sectional area in the orbit of the junctions between said first and said second magnetic structures for causing an increase of the resistance against electrical currents induced in each lamination of said second magnetic structure, and covers of magnetisable material closing the open sides of said second magnetic structure defining therewith a housing for said electrical coil. 5. A coil structure comprising a magnetizable core structure having an elongated central core member, an electrical coil structure surrounding said central core member and a second core member comprising a plurality of laminations defined by super-imposed strip layers of magnetic ribbon facing said central core member with their broad sides, said second core member being in the form of a structure defining a loop encircling said central core member, said second core member having portions in the orbit of its junction with said central core member of reduced cross-sectional area thereby forming apertured portions which extend to at least near one edge of said second core member.

6. A coil structure comprising a magnetizable core structure having an elongated central core member, an electrical coil structure surrounding said central core member and a second core member comprising a plurality of laminations defined by super-imposed strip layers of magnetic ribbon facing said central core member with their broad sides, said second core member being in the form of a structure defining a loop encircling said central core member, said second core member having portions in the orbit of its junction with said central core member being of reduced cross-sectional area, said loop having a height corresponding substantially to the diameter of said electrical coil, and covers of magnetisable material at each end of said loop for defining together therewith a magnetisable housing for said electrical coil.

7. An induction coil structure comprising in combination: a coil winding, an elongated straight laminated core member extending through the interior of said coil winding and having exposed end surfaces, a secondary core member in the form of a coiled band of magnetizable material wound in the shape of a member being of substantially rectangular configuration having side walls and end walls defining together a tubular space with said band facing the interior of said space with its broad side, means for supporting said primary core extending between opposite end walls inside said secondary core member, and slots in said coiled band in the orbit of the ends of said primary core member, the slots in the different layers of said secondary core member being substantially in alignment.

8. An induction coil structure comprising in combination: an electrical coil winding structure, an elongated substantially straight laminated primary core member extending through the interior of said coil winding structure and haivng exposed end surfaces, a secondary core member surrounding said primary core member and said coil winding as a mantle, said secondary core member comprising a plurality of laminations of super-imposed layers of a continuous magnetic ribbon defining a loop in which said primary core extends between opposite end walls,

. Z and slots in said ribbon in each of said layers of said loop in the orbit of the junction between the end surfaces of said primary core member and said secondary loop to provide reduced cross-sectional areas, said slots extending to at least near one edge of said ribbon.

9. In an induction coil having a straight elongated core member having end portions and a coil winding surrounding said straight core member: an exterior core member comprising a winding of a plurality of turns of magnetizable strip material defining by the superposed layers of said band material a hollow core mantle having the layers thereof arranged with their broad sides facing the interior of said mantle, means holding said straight core in position extending across said interior of said mantle with the end portions of said straight core facing the interior of said mantle, and elongated slots in the layers of said mantle in the area thereof which faces the end portions of said straight core member, said slots extending at a substantial angle to the direction of the band and over a considerable portion of the width of the band near at least one of the edges thereof.

10. An electromagnetic apparatus comprising a coil winding of insulated wire, an interior substantially rodlike magnetic core member having opposite ends, means for supporting said coil winding on said rod-like member, an exterior core mantle defininga tubular wall structure encircling a hollow space of substantially rectangular configuration having side walls and end walls, the walls having a plurality of layers of strip material arranged with their broad sides facing the hollow space of said core mantle, means supporting said interior core extending between said end walls in said hollow space, and at least one slot in each of said mantle layers at the orbit which faces said rod-like core member end surfaces effectively subdividing the orbits of said mantle layers into restricted zones, and cover plates of magnetizable material in engagement with the ends of said wall structure sep arated therefrom by air gaps.

11. An electromagnetic apparatus comprising a coil Winding of insulated wire, an interior, substantially rod- .like, magnetic core member having opposite ends, means for supporting said coil Winding on said rod-like member, an exterior core mantle defining a tubular wall structure encircling a hollow space of substantially rectangular configuration having side walls and end walls, the walls having a plurality of layers of strip material arranged with their broad sides facing the hollow space of said core mantle, means supporting said interior core extending between said, end walls in said hollow space and at least one slot in each of said mantle layers at the orbit which faces said rod-like core member end surfaces elfectively subdividing the orbits of said mantle layers into restricted zones and covers of magnetisable material closing the hollow space encircled by said exterior core :mantle.

12. An induction coil comprising a straight magnetic core, having end surfaces, a coil Winding encircling said straight core leaving its end surfaces exposed, a laminated tubular core structure defining an exterior core mantle and having a plurality of layers of strip material arranged with their broad sides facing the interior of said tubular core member, means supporting said straight core as an interior leg inside the space of said tubular core, slots in a plurality of said tubular core strips at the orbit of said straight core end surfaces effectively subdividing the rality of strip layers of magnetizable material arranged with their broad sides facing said hollow space and having at least two elongated slots in the ends of said exterior core member at which said straight core member terminates to effectively subdivide the ends of said exterior core member into restricted zones.

14. In an induction coil having a straight elongated core member and a coil winding surrounding said straight. core member: an exterior core member in the form of a wall structure of rectangular configuration encircling a hollow space across which said straight core member extends, said wall structure being in the form of a plurality of strip layers of magnetizable material arranged with their broad sides facing said hollow space and having at least one elongated slot in each of said strip layers in the orbit of the ends of said straight core member to thereby effectively subdivide the orbits of said strip layers into restricted zones, said slots being in alignment to define channels extending through said wall structure.

15. An electromagnetic apparatus comprising a coil winding of insulated wire, an interior, substantially rodlike, magnetic core member having opposite ends, means for supporting said coil winding on said rod-like member, an exterior core mantle defining a tubular wall structure encircling a hollow space of substantially rectangular configuration having side walls and end walls, the end walls having a plurality of layers of strip material arranged with their broad sides facing the hollow space 'of said core mantle, means supporting said interior core extending between said end walls in said hollow space,

and at least one slot in each of said mantle layers at the orbit of said rod-like core member end surfaces effectively subdividing said orbits of said mantle layers into restricted zones, said slots being in substantial alignment and defining channels through said mantle extending from the exterior thereof to said end surfaces of said straight core.

16. An electromagnetic apparatus according to claim 15, wherein two slots are provided in each of said mantle layers spaced substantially the same distance at each side of the geometric axis of said interior magnetic core member, and the slots extending from an edge of the strip mat'erial over a distance greater than half the width of the strip material.

17. An electromagnetic apparatus comprising a coil winding of insulated wire, an interior, substantially rodlike, magnetic core member having opposite ends, means for supporting said coil winding on said rod-like member, an'exterior core mantle defining a tubular wall structure encircling a hollow space of substantially rectangular configuration having side walls and end walls, the walls having a plurality of layers of strip material arranged with their broad sides facing the hollow space of said "core mantle, means supporting said interior core extending between said end walls in said hollow space, and at least one slot in each of said mantle layers at the orbit of said rod-like core member end surfaces effectively subdividing said orbits of said mantle layers into restricted zones.

18. An induction coil comprising a straight magnetic core, having end surfaces, a coil Winding encircling said straight core leaving its end surfaces exposed, a laminated tubular core structure having a plurality of layers of strip "material arranged with their broad sides facing the in terior of said tubular core structure, means supporting said straight core as an interior leg inside the space of said tubular core, and slots in a plurality of said tubular core strips at the orbit of said straight core end surfaces.

19. In an induction coil having a straight elongated core member and a coil winding surrounding said straight core member, an exterior core member in the form of a wall structure encircling a hollow space across which said straight core member extends, said wall structure being in the form of a plurality of strip layers of magnetizable material arranged with their broad sides facing said hollow space and having elongated slots in the orbit of the ends of said straight core member.

20. In an induction coil, the combination according to claim 19, wherein at least two of said slots are provided in the orbit of each strip layer within the area of said orbit and disposed on both sides of the geometrical axis of said elongated core member and approximately the same distance from said axis.

21. In an induction coil, the combination according to claim 19, wherein said slots extend from an edge of the strip layers for such a distance that they also extend over a substantial portion of the surface of said elongated core member.

22. An induction coil structure comprising a flux generating portion including an inner core embraced by a coil and a flux distributing portion including an outer core structure having end members of sheet material which face with the broad side thereof the ends of said inner core, the end members of sheet material being provided with edge portions electrically confining said end sheet material, and means in the form of slots within said end sheet material to reduce the eddy currents therein, said slots effectively subdividing said end sheet material into electrically conductive substantially spaced portions of reduced area operable to substantially confine eddy currents in said end sheet material generated by the flux generating core portion to flow within the end sheet material only substantially within each of said reduced area portions.

23. An induction coil structure comprising a flux generating portion including an inner core embraced by a coil and a flux distributing portion including an outer core structure having end members of sheet material which face with the broad side thereof the ends of said inner core, the end members of sheet material being provided with edge portions electrically confining said end sheet material, and means in the form of slots extending from a region of an edge portion toward an area of greater fiux density within said end sheet material to reduce the eddy currents therein, said slots effectively subdividing said end sheet material into electrically conductive substantially spaced portions of reduced area operative to substantially confine eddy currents in said end sheet material generated by the flux generating core portion to flow within the end sheet material only substantially Within each of said reduced area portions.

24. An induction coil structure comprising a magnetic flux generating portion including an inner core embraced by a coil and a fiux distributing portion including an outer core structure having ends of sheet material facing with their broad sides the ends of the inner core, each end of sheet material of said outer core structure facing a respective end of said inner core being provided with at least one free edge portion defining the same and provided with eddy-current-reducing subdividing means effectively subdividing, with respect to eddy currents, said end sheet material inclusive the zone of at least one edge portion to provide Within said end sheet material substantially spaced portions of reduced area, said subdividing means extending sufiiciently across said end sheet material to be operative to confine eddy currents within said sheet material generated by said flux generating portion to flow in said end sheet material substantially only within individual ones of said subdivided portions.

25. An induction coil comprising an inner core of magnetizable material having end surfaces and embraced by a coil of electrically conductive material, an outer core of magnetic material forming in conjunction with the inner core a closed magnetic circuit embracing the coil, said outer core having end surface means defined at least in part by free edge means, at least one end surface of the inner core being disposed opposite the end surface means of said outer core in flux interlinking relationship therewith, and means in the form of slot means extending through a substantial number of layers of lamination of said end surface means from the region of at least one of said edge means toward the central region thereof and so arranged and constructed as to reduce the cross,

sectional area of said end surface means into a plurality of eddy current confining zones adjacent the ends of the inner core to thereby limit the flow of eddy currents substantially to individual zones.

26. An induction coil comprising an inner core of magnetizable material having end surfaces and embraced by a coil of electrically conductive material, an outer core of magnetic material forming in conjunction with the inner core a closed magnetic circuit embracing the coil, said outer core having end surface means defined at least in part by free edge means, at least one end surface of the inner core being disposed opposite the end surface means of said outer core in flux interlinking relationship therewith, the outer core being laminated with the planes of the lamination substantially at right angle to the medial plane of the inner magnetic core, and means in the form of slot means extending through a substantial number of layers of lamination of said end surface means from the region of at least one of said edge means toward the central region thereof and so arranged and constructed as to reduce the cross sectional area of said end surface means into a plurality of eddy current confining zones adjacent the ends of the inner core to thereby limit the flow of eddy currents substantially to individual zones.

27. An induction coil structure comprising an inrier core of magnetizable material having end surface means and adapted to support thereon a coil of electrically conductive material so as to produce magnetic flux within said inner core, and an outer core of magnetic material forming in conjunction with the inner core a substantially closed magnetic circuit, said outer core also including end surface means facing the end surface means of said inner core in magnetic flux interlinking relationship therewith in such a manner as to normally induce eddy currents within the end surface means of said outer core, said outer core end surface means being provided with free edge portion means defining at least in part the end surface means thereof, the end surface means of said outer core being provided with eddy current limiting means effectively subdividing the outer core end surface means into a plurality of subdivided zones operative to constrain or limit the closed paths of eddy currents within said outer core end surface means to the area of individual zones.

28. An improved induction coil structure adapted to operate under varying operating conditions such as a ballast for a luminous tube, and which has reduced electrical power losses improving its electrical efficency, comprising inner core means embraced by a coil and having at least one end and outer core means which together with said inner core means forms a substantially closed magnetic circuit for the flux produced by current flowing through said coil, at least those parts of said outer core means which are located adjacent the end of said inner core means being of sheet-like material having at least one free edge portion with the inner face of said sheet like material substantially facing the end of said inner core means, at least most of said parts being provided with eddy-current confining means extending substantially inwardly from the area of said edge portion for causing the eddy current generated in the respective part by the face Wise entry of magnetic flux from the adjacent end of said inner core means to circulate in a plurality of circuits, each circuit being substantially separated from the other circuit by the said means and the eddy current in each circuit being generated by a part only of said flux.

29. An improved induction coil structure adapted to operate under varying operating conditions, for example as a ballast for a luminous tube, and which has reduced electrical power losses improving its electrical eificiency, comprising an inner core embraced by a coil and an outer core structure which together with the inner core forms a closed magnetic circuit embracing the coil, at least those parts of the outer core structure which are located adjacent the ends of the inner core being of sheet material with inner faces thereof facing towards said ends, each said part being provided with means extending substantially from anedge of the said material inwardly of the said edge in the form of a slot for causing the eddy currents generated in the said part by the facewise entry of magnetic flux from the adjacent end of the inner core to circulate in two or more circuits, each circuit being substantially separated from the other circuit by the said means and the eddy current in each circuit being generated by a part only of the said flux.

30. An induction coil structure comprising a magnetic flux generating portion including an inner core embraced by a coil and a flux distributing portion including an outer core structure having ends of sheet material facing with their broad sides the ends of the inner core, each end of sheet material of said outer core structure facing a respective end of said inner core being provided with at least two free edge portions defining the same and provided with eddy-current-reducing subdividing means effectively subdividing, with respect to eddy currents, said end sheet material inclusive the zone of at least one edge portion to provide within said end sheet material substantially spaced portions of reduced area, said subdividing means extending sufficiently across said end sheet material to be operative to confine eddy currents within said sheet material generated by said flux generating portion to flow insaid end sheet material substantially only within individual ones of said subdivided portions. 

18. AN INDUCTION COIL COMPRISING A STRAIGHT MAGNETIC CORE, HAVING END SURFACES, A COIL WINDING ENCIRCLING SAID STRAIGHT CORE LEAVING ITS END SURFACES EXPOSED, A LAMINATED TUBULAR CORE STRUCTURE HAVING A PLURALITY OF LAYERS OF STRIP MATERIAL ARRANGED WITH THEIR BROAD SIDES FACING THE INTERIOR OF SAID TUBULAR CORE STRUCTURE, MEANS SUPPORTING SAID STRAIGHT CORE AS AN INTERIOR LEG INSIDE THE SPACE OF SAID TUBULAR CORE, AND SLOTS IN PLURALITY OF SAID TUBULAR CORE STRIPS AT THE ORBIT OF SAID STRAIGHT CORE END SURFACES. 